Open A String Buzzing

HELP ME!
I was cleaning my bass tonight and I ran into a problem. I put my strings back into the bass and the A string started buzzing. It sounds fine fretted. I already tried taking it and putting it back in (3 times). What's wrong?

Is it audible through an amp? If not, you should be okay, my bass does that, infact quite a few basses do (well at least in my experience) it shouldn't be anything to worry about. If your really concerned, you should probably get it checked out.

If you just changed strings, how far down the post are the windings? If they are too high, chances are they are not getting enough break over the nut, and it is vibrating in the nut. I had this problem, along with many others, and usually that is the problem. Also, the nut may be worn down a little on the A. When fretted, you are giving the string more of a break angle over the nut, so it doesn't vibrate.

Try rewinging the string lower on the post. If that doesn't work let me know.